Mark Wefers Bettink

A monitor for Cellular Oxygen METabolism (COMET): monitoring tissue oxygenation at the mitochondrial level 3 55 to organ function still needs further investigation but important is that also changes in cutaneous mitochondrial oxygen consumption correlate with ODR changes in other organs and tissues [17]. The ODR from the dynamic measurement 6.3 mmHg.s -1 in this example corresponds with previous data from healthy volunteers 5.8 ± 2.3 mmHg.s -1 [29] . In this paper we describe the case of clonidine given to a patient within a short period of time. This incidental finding occurred during an ongoing neurosurgery feasibility study. This patient group was chosen because in general we aimat hemodynamic stability during surgery over a longer time period. Therefore the incidental finding of an abrupt drop in cutaneous mitoPO 2 after a bolus clonidine came clearly forward. In line with clinical observations the effects of clonidine administration resulted in initial vasoconstriction and subsequent vasodilatation after a couple of minutes. Using online monitoring we observed a direct decline inmicrovascular blood flow and velocity, followed by an increase in flow and velocity as measured by the O2C. With the change of flow and velocity a decrease in the oxygen supply to the tissue is expected. In consequence a decrease in mitoPO 2 of 30mmHg is observed as measured by the COMET. However, interestingly, the capillary-venous oxygen saturation measured by the O2C did not show any decrease in the minutes following clonidine administration. Two main phenomena could explain the unchanged capillary-venous oxygen saturation. First, the SO 2 is measured with the absorbance of visible light; the velocity and flow are measured with the hemoglobin laser doppler frequency shift. The different wavelengths of light, giving a different tissue penetration and therefore measurement compartment [30, 31], used in these techniques may explain why a difference in flow but not of SO 2 after clonidine administration could be observed. A second explanation could be a total stop flow of some capillaries. The part of the capillary tree without flow does not contribute to venous-capillary saturation. Thus, the oxygen extraction in the measurement volume stays the same. For the COMET measurements heterogeneity of the oxygen content in the measurement volume could be demonstrated [8]. Therefore a heterogeneous bimodal distribution could explain the decrease in flow and a constant oxygen capillary venous saturation. The reader should keep in mind that for practical reasons the O2C and COMET measurements were performed in close proximity of each other, but not in exactly the same area of the skin. However, we do think that for a valid comparison of the measurements the fact that SO 2 and mitoPO 2 were measured at the same depth in skin is of more importance. Based on our findings it is clear that measuring oxygen availability directly at cellular level provides complementary data and new insight.